Light emitting diodes (LEDs) are an important class of solid-state devices that convert electric energy to light. Improvements in these devices have resulted in their use in light fixtures designed to replace conventional incandescent and fluorescent light sources. The LEDs have significantly longer lifetimes and, in some cases, significantly higher efficiency for converting electric energy to light.
The cost and conversion efficiency of LEDs are important factors in determining the rate at which this new technology will replace conventional light sources and be utilized in high power applications. Many high power applications require multiple LEDs to achieve the needed power levels, since individual LEDs are limited to a few watts. In addition, LEDs generate light in relatively narrow spectral bands. Hence, in applications requiring a light source of a particular color, the light from a number of LEDs with spectral emission in different optical bands is combined or a portion of the light from the LED is converted to light of a different color using a phosphor. Thus, the cost of many light sources based on LEDs is many times the cost of the individual LEDs. To reduce the cost of such light sources, the amount of light generated per LED must be increased without substantially increasing the cost of each LED and without substantially lowering the conversion efficiency of the individual LEDs.
The conversion efficiency of individual LEDs is an important factor in addressing the cost of high power LED light sources. The conversion efficiency of an LED is defined to be the electrical power dissipated per unit of light that is emitted by the LED. Electrical power that is not converted to light in the LED is converted to heat that raises the temperature of the LED. Heat dissipation places a limit on the power level at which an LED operates. In addition, the LEDs must be mounted on structures that provide heat dissipation, which, in turn, further increases the cost of the light sources. Hence, if the conversion efficiency of an LED can be increased, the maximum amount of light that can be provided by a single LED can also be increased, and hence, the number of LEDs needed for a given light source can be reduced. In addition, the cost of operation of the LED is also inversely proportional to the conversion efficiency. Hence, there has been a great deal of work directed to improving the conversion efficiency of LEDs.
For the purposes of this discussion, an LED can be viewed as having three layers, the active layer sandwiched between a p-doped layer and an n-doped layer. These layers are typically deposited on a substrate such as sapphire. It should be noted that each of these layers typically includes a number of sub-layers. The overall conversion efficiency of an LED depends on the efficiency with which electricity is converted to light in the active layer. Light is generated when holes from the p-doped layer combine with electrons from the n-doped layer in the active layer.
The amount of light that is generated by an LED of a particular size can, in principle, be increased by increasing the current passing through the device, since more holes and electrons will be injected per unit area into the active layer. However, at high current densities, the efficiency with which holes combine with electrons to produce light decreases. That is, the fractions of holes recombine without producing light increases. Hence, as the current is increased through the device, the efficiency decreases and the problems associated with high operating temperatures increase.